This invention relates to nuclear reactor fuel assemblies and is concerned mainly with multi-pin fuel assemblies with the pins contained within a hexagonal tubular wrapper and stabilised by a series of spaced grids. Such assemblies find one use in liquid metal cooled fast reactors.
Conventionally, assemblies having a hexagonal wrapper do not have a fuel pin at the corners of the hexagonal shape. This is brought about as either cooling is difficult at this locality or the locality is required for structural members such as corner posts to hold the grids apart (see for example GB Pat. No. 2,079,033B, item 17 FIG. 2). There is a preference to have a fuel pin at this locality in order to maximise power output and the present invention aims to meet this preference.
Further, it is now being observed that a wrapper which serves only a shrouding function with a minimum load-bearing function can have advantages as it allows a wider choice of materials for the wrapper and in particular allows the choice of materials which do not retain good structural characteristics under irradiation. Such materials, for example those that embrittle seriously with irradiation, would previously have been rejected for use as they would not tolerate welding or drilling for joining with other members (such as grids).
It is known, in water-cooled reactors, to employ longitudinal support members for the fuel pin spacing members (see British Pat. No. 1137621) but these are located at the corners of the wrapper or outer casing and would be unacceptable in the kind of fuel assembly with which the present invention is concerned because coolant flow at the corners of a hexagonal array of fuel pins would be unduly restricted.